1. Technical Field
The present invention relatives to a mixer and more particularly to a mixer with calibration circuit to improve LO leakage.
2. Description of the Prior Art
Wireless communication has been widely applied in various applications. Typically, each wireless communication device requires a radio transceiver (i.e., a receiver and a transmitter). With the demand for enhanced performance, a direct conversion technology has been developed for size, cost and lower power consumption.
Referring to FIG. 1, a circuit diagram of a conventional mixer is shown. As illustrated, the conventional mixer 10 comprises a gm stage block 11, a LO switch stage block 13, and an output load stage block 15. A RF input signal is amplified in the gm stage block 11, down-converted to an IF current signal in the LO switch stage block 13, and then converted to a voltage signal by the output load stage block 15.
The gin stage block 11 comprises transistors M1 and M2, and the RF input signal, such as two differential input signals Vp and Vn, are inputted transistors M1 and M2 of the input gm stage block 11. The gm stage block 11 is capable of perform a voltage, such as Vp and Vn, to current conversion, and affects the gain and linear of the mixer 10.
The LO switch stage block 13 comprises transistors M3, M4, M5 and M6 and form a multiplication function, multiplying the linear RF signal current from M1 and M2 with the LO signal applied across M3 to M6 which provide the switching function. M1 and M2 provide +/−RF current and M3 and M4 switch between them to provide the RF signal or the inverted RF signal to the left hand load. M5 and M6 switch between them for the right hand load. The output load stage block 15 comprise two load resistors R1 and R2 form a current to voltage transformation giving differential output IF signals.